Vector velocimeter

ABSTRACT

A velocimeter having: a source of coherent radiation directed to irradiate a surface from which the relative velocity is to be measured; a receiver for the resulting reflection of coherent radiation from said surface, the receiver including a receiving aperture and a photodetector responsive to the reflected radiation passing through the aperture; a drive to translate the aperture, and; circuitry to indicate the frequencies of the signals generated by the photodetector when the aperture is stationary in relation to the photodetector and when the aperture is translated in relation to said photodetector. The velocimeter determines the magnitude and the direction of the velocity vector it measures. A navigation velocimeter, using the described velocimeter, is disclosed for use in an aircraft to determine the direction and magnitude of the velocity vector of the aircraft with respect to the ground. The aperature hereinabove mentioned is preferably a plurality of slits, alternately transparent and opaque, such as in a transmission type optical diffraction grating.

United States Patent [191 Blau et al.

[ June 5, 1973 [54] VECTOR VELOCIMETER [75] Inventors: Donald Z. Blau,City Island; Jesse C. Kaufman, Yorktown Heights, both of NY.

[73] Assignee: The Singer Company, New York,

22 Filed: Feb.22, 1971 21 Appl.No.: 117,349

'[5'2] U.S'.' Cl..... .....'.".;.'1':;;T ..'.....356/28 [51] Int. Cl...G0lp 3/36 [58] Field of Search ..356/28 [56] References Cited UNITEDSTATES PATENTS 3,432,237 3/1969 Flower et al ..356/28 2,772,479 12/1956Doyle ..356/28 3,511,150 5/1970 Whitney et a1. ..356/28 2,942,119 6/1960King et al. ..356/28 Primary Examiner-Benjamin A. Borchelt AssistantExaminer-S. C. Buczinski Att0rney-S. A. Giarratana and Thomas W. Kennedy[57] ABSTRACT A velocimeter having: a source of coherent radiationdirected to irradiate a surface from which the relative velocity is tobe measured; a receiver for the resulting reflection of coherentradiation from said surface, the receiver including a receiving apertureand a photodetector responsive to the reflected radiation passingthrough the aperture; a drive to translate the aperture, and; circuitryto indicate the frequencies of the signals generated by thephotodetector when the aperture is stationary in relation to thephotodetector and when the aperture is translated in relation to saidphotode-' tector. The velocimeter determines the magnitude and thedirection of the velocity vector it measures. A navigation velocimeter,using the described velocimeter, is disclosed for use in an aircraft todetermine the direction and magnitude of the velocity vector of theaircraft with respect to the ground. The aperature hereinabove mentionedis preferably a plurality of slits, alternately transparent and opaque,such as in a transmission type optical diffraction grating.

13 Claims, 9 Drawing Figures PATENTEDJUH 5l975 3 73T 233 SHEET 10F 4 1Tn I4 24 LASER ,7 s m FREQ. .4 FIG. 1 Q5 I u 22 3 't:: p'

Vg U9 l2 FIG. 2. BACKSCATTERED RAD'AT'ON BUCKET READ COUNTER our FIG. 4.44

O I \J 45% l 29' 1 W" a ll AI I f & NH Q:.

42 4s 43 \ZW\ INVENT OR DONALD Z. BLAU 8| JESSE C. KAUFMAN TTORNEYSPATENTEDJUH 5 I973 SHEET 3 BF 4 READ BUCKET coumsn OUT AGC AMPLIFIERBACK- SCATTE RED REFERENCE- NORTH INVENTOR DONALD Z. BLAU 8 JESSE C.KAUFMAN VECTOR VELOCIMETER CROSS-REFERENCE TO RELATED APPLICATION US.patent application Ser. No. 86,897 entitled Vector Velocimeter(Direction Indicating Velocimeter) by R.A. Flower et a1, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to velocity measuring devices and more particularly to devicesin which a frequency characteristic proportional to the relativevelocity and direction between a body and a target is derived from waveradiation projected from the body to the target,

'reflected therefrom, and returned to the projecting body.

2. Description of the Prior Art The measurement of the relative velocitybetween two objects without any physical contact between them isfrequently desirable. Various devices using different portions of theelectromagnetic spectrum to accomplish this purpose have been devised.Such devices are disclosed in US. Pat. No. 3,432,237 issued Mar. 11,1969 to the assignee of the present application. That patent discloses asource of radiation, such as a laser, for directing a substantiallymonochromatic beam toward a reference surface. The reflected radiationis received by a device where it is passed through an aperture or aplurality of slits located near the source and received by aphotomultiplier tube which has its anode connected to a frequency meter.The output of the frequency meter is a function of the magnitude of therelative velocity between the radiation source and a reflecting surface.The patentees in the above-identified patent also contemplated thatother wave sources of limited band width yielding radio waves, sound orlight could be used. According to another aspect of their invention, theaforementioned patentees disclose a pair of the above-describedreceiving devices, each device having its velocity axis inclined at anequal angle on opposite sides of a vehicle. The output from thereceiving devices is processed by a suitable computer to obtain themagnitudes of the velocity and the drift angle of the vehicle.

While the velocity measuring devices of the aforementioned patent arereliable in their operation and fulfill their intended purpose, it hasbeen found that it would be desirable to provide a velocity measuringdevice of a similar character but also having the capability to indicateboth the magnitude and direction of the velocity vector. In the case ofa vehicle, it would be desirable to provide a velocimeter to indicatethe phase of the drift angle of the vehicle as well as the magnitude ofsuch angle.

SUMMARY OF THE INVENTION An object of the invention is to provide avelocimeter for determining the magnitude and direction of the relativevelocity between two objects which are not in physical contact or wheresuch physical contact as may be present does not lead to accuratevelocity determination.

' Another object of the invention is to provide a velocimeter fordetermining the magnitude of the velocity and drift angle of a vehicle,the phase of the drift angle, and the absolute heading of said vehicle.

Other objects and advantages of the present invention will be apparentto those skilled in the art by description of the preferred embodimentsof the invention which follows.

According to one aspect, the invention comprises a velocimeter having asource of coherent radiation directed to irradiate a surface from whichthe relative velocity is to be measured. The velocimeter also has lightreceiving means to receive the resulting reflection of the coherentradiation from the surface. The light receiving means includes areceiving aperture and detecting means responsive to the intensity ofthe received radiation passing through the aperture. The velocimeterdetects the direction of the relative velocity by providing apredeterminedrelative motion between the aperture and the reflectedradiation received by the light receiving means. When the detectingmeans and receiving aperture are relatively stationary, the reflectedradiation passed through the aperture generates a signal with afrequency proportional to the magnitude of the relative velocity betweenthe velocimeter and the surface. When the receiving aperture istranslated with respect to the reflected radiation in a known directionand at a speed less than the unknown relative velocity to be measured,the reflected radiation passed through the aperture generates a signalwith a frequency component due to the relative velocity of thevelocimeter and the surface and another component due to the translationof the receiving aperture. If the frequency of the signal generated isgreater than the frequency generated when the detecting means andaperture are relatively stationary, then the direction of the velocityvector is opposite to the direction in which the receiving aperture istranslated with respect to the reflected radiation. Conversely, if thefrequency of the signal generated is less than the frequency generatedwhen the detecting means and aperture are relatively stationary, thenthe direction of the velocity vector is the same as the direction inwhich the receiving aperture is translated.

According to another aspect, the invention relates to a velocimeter fordetermining the relative velocity vector between a target and a vehicle.The velocimeter has a source of coherent radiation directed to irradiatea surface on the target. It also has light receiving means to receivethe resulting reflection of coherent radiation. This light receivingmeans includes at least first and second gratings having theirrespective grating lines oriented at first and second known angles withthe longitudinal axis of the vehicle, respectively. The light receivingmeans also includes first and second detecting means responsive to theintensity of the received radiation passing through the first and secondgratings, respectively. The light receiving means also includestranslating means to provide a predetermined relative motion between thesecond grating and the reflected coherent radiation received by thelight receiving means. The velocimeter according to this aspect of theinvention can be used to determine the magnitude of the velocity vectorand the drift angle as well as the phase of the drift angle. When thedetecting means and gratings are relatively stationary, the reflectedradiation passed through the gratings generates respective signals withfrequencies proportional to the magnitude of the relative velocitycomponents along axes perpendicular to the lines of the respectivegratings. These respective signals are combinable according totrigonorection perpendicular to the lines of the second grating.Vectorial laws can be used to determine the phase of the drift angleonce the latter direction is known.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representationof one embodiment of the present invention.

FIG. 2 is a schematic representation of details of an embodiment similarto that of FIG. 1.

FIGS. 3A and B are schematic representations of another embodiment ofthe present invention.

FIG. 4 is an oblique view, with portions broken away, of further detailsof the embodiment of FIG. 3A.

FIG. 5 is a schematic representation of another embodiment of thepresent invention.

FIG. 6 is a schematic plan representation of an airplane in horizontalflight.

FIG. 7 is a schematic representation of an embodiment of the presentinvention when used on a vehicle such as an airplane.

FIG. 8 is a schematic representation of the angle of grating orientationaccording to one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 illustrate oneembodiment of the present invention. The velocimeter shown by FIGS. 1and 2 are improvements of the velocimeter disclosed in theaforementioned US. Patent and applicants intend that the disclosuretherein be incorporated in this description. Referring to FIG. 1, alaser 11 or other monochromatic source of electromagnetic radiationprojects a beam of radiation 12 toward a reflecting surface 13 of thetarget. The laser 11 is mounted on a supporting structure generallyindicated by the dotted rectangle 14 which may be a vehicle or a fixedsupporting structure. The beam 12, in impinging on the surface 13 over afinite area having a diameter d produces a backscattered pattern 16. Asdescribed in the aforementioned US. Patent, the radiation backscatteredfrom a target surface exhibits a lobed or granular pattern. Thisrequires that the radiation source be at least partially coherent(preferably highly coherent) and that the target surface be to somedegree non-specular at the radiation wavelength. This is easily met whenlaser sources illuminate most surfaces since the dimensions of thesurface elements are large with respect to the radiation wavelength,thus, the scattering centers reradiate with mutually random phases. Itshould be understood that applicants velocimeter may use variouswavelengths of radiation provided the above conditions are met so as toobtain a granular backscattered pattern. The velocimeter of FIG. 1 alsohas a receiver mounted on the supporting structure 14. The receivercomprises a I the supporting structure relative to the reflectingsurface 13. The output of the photodetector is impressed on a frequencymeasuring device 21 which may be any well-known frequency meter. To thispoint, the elements of FIG. 1 are conventional and correspond to thevelocimeter disclosed by the aforementioned patent.

FIG. 1 illustrates the improvement according to one embodiment of thepresent invention wherein the optical diffraction grating 19 is in theform of an endless belt and is provided with means to translate it at aknown velocity, V,,, during part of the time. The translating meanscomprises a driven pulley 22 and an idler pulley 23. Pulley 22 isrotated by a sequential drive 24 so that grating 19 is moved at V, partof the time and remains stationary with respect to photodetector 17 theremainder of the time. The significance of such grating translation willbe described below.

FIG. 2 is a schematic illustration of a preferred receiver embodimentwhich is generally described above with reference to FIG. 1. Accordingto FIG. 2, the signals from the photodetector are fed to an amplifier,then to a bucket counter whose output voltage is proportional to thefrequency of the signals from the photodetector. When grating 19 isstationary in relation to photodetector 17, the frequency of the signalfrom the photodetector is f,; when the grating 19 is translated inrelation to the photodector 17, the frequency of the signal from thephotodetector is f,,,. The read out means indicates the magnitude anddirection of the velocity vector V in a manner now to be described.

The embodiment of FIGS. 1 and 2 includes an optical diffraction grating19 having grating line spacings, G, and being in the form of an endlessloop which is translated at a fixed velocity V, (of known direction andmagnitude) in front of a stationary photodetector 17. For a stationarygrating having a particular space frequency, SF, for a target moving ata fixed velocity V normal to the grating linesand for a given systemoptical leverage OL, the velocimeter generates a spectrum whose centerfrequency f is related to this relative velocity by i f. (VT) where OLoptical leverage which is the ratio of granule velocity in the receivingplane to that of the target velocity. The numerical quantity assigned toCL is determined by the geometrical properties of the transmitter beamas discussed in U.S. Pat. No. 3,432,237;

SF space frequency which is the reciprocal of the line spacing, G, ofthe optical grating lines.

If the grating is translated in front of the photodetector 17 at a fixedvelocity V the center frequency f generated by the detector will bealtered depending upon the relative velocity between the target andgrating, and upon the target velocity direction and direction of gratingmotion. The velocity of the grating loop must be such that the resultantchange in frequency Af, given by is detectable. This constitutes a lowerlimit for Af. The induced change in frequency resulting from the gratingmotion must not exceed the magnitude of the generated center frequencyf,, when the grating is stationary. This represents the upper limit forAf.

To determine the direction of relative targetsupporting structure motion(for a known optical leverage), it is only necessary to observe thedirection of the frequency change, with respect to the frequencygenerated when the grating is stationary, within the limits previouslydescribed. For example, for a grating velocity V,,, as shown in FIG. 1,the frequency will decrease and thus indicate that the direction of thevector V is the same as the known direction for vector V,,.

FIG. 3A illustrates another embodiment of the present invention. Thisembodiment also generates signals having frequencies f, and f,,analogous to those signals described above in connection with FIGS. 1and 2. In the latter figures, the f and f,,, signals were generated byalternately changing the grating 19 from a stateof no movement relativeto the photodetector 17 to one of uniform grating motion. This providesa continuous readout of both target velocity and direction. According toFIG. 3A, two photodetectors 27, 27' are used in the receiver and a fixedgrating 29 is placed in front of photodetector 27 and a continuouslymoving grating 29 is placed in front of the photodetector 27'. An electronic switch 30 alternately feeds signals having frequencies f, andf,,, to read out instrumentation similar to that described in connectionwith FIGS. 1 and 2. FIG. 3B shows details of a typical circuit such ascould be used in FIG. 3A. The magnitude of Af is given by I Af I =f,f,,, and is indicated by the magnitude of deflection of the readoutammeter M (initially adjusted at the center of the scale for AH) whilethe sign of Af is given by the direction of deflection of the meterpointer which responds to the current produced due to the charging anddischarging of capacitor C.

FIG. 4 illustrates a preferred translating means for the grating 29' ofthe embodiment of FIG. 3A. The grating 29' is supported by a drivensprocket pulley 42 and an idler pulley 43. Pulley 42 is driven by amotor 44 through a suitable gear train. The backscattered radiation 46is reflected through the grating 29 by a 45 front surface mirror orprism 47; then through a limiting aperture 48 to the photodetector.

FIG. 5 illustrates another embodiment of the present invention. Thisembodiment also generates signals having frequencies f, and f,analogous, to those signals described above in connection with FIGS. 1and 2. According to FIG. 5, a single photodetector is used and oneportion 57 is masked by a fixed grating 59 and another portion 57 ismasked by a continuously moving grating 59'. The gratings are precededby an optical chopper e.g., rotary shutter, which alternately allowsbackscattered radiation to pass through gratings 59, 59' andtophotodetector portions 57, 57. Rather than using a singlephotodetector with two portions 57, 57', two separate photodetectorscould be used.

A further aspect of applicants invention relates to a velocimeter whichhas application to navigation systems. It enables a moving vehicle todetermine its velocity vector (ground speed and drift angle) relative toa stationary, nonspecular target. In one embodiment, FIG. 7, thevelocimeter located aboard the vehicle contains a highly coherent source71 which illuminates the target 73, three detectors which intercept andoperate upon a portion of the radiation 76 backscattered from the targetand electronic circuitry which combines the outputs of the detectors insuch a manner as to provide the desired information, namely, groundspeed and drift angle. The radiation 76 backscattered from a targetsurface exhibits a granular structure as described above.

For convenience in discussing the velocimeter, an airplane in horizontalflight shall be considered such as depicted in FIG. 6. For the purposeof navigation, the ground speed, V, and the drift angle, 9 must bedetermined. FIG. 7 illustrates one embodiment of applicants inventionwhereby these two parameters can be determined.

According to the embodiment of FIG. 7, three detec tors are provided: DD and D whose outputs are related to the components of V seen in FIG. 6,namely V (velocity along heading), V, (velocity cross heading), and 0(drift angle). A gyro provides the necessary heading reference, V 7 a rThe outputs of detectors D and D provide signals whose centerfrequencies are proportional to the magnitudes |V,,,| and ]V,,,,]respectively. Detectors D and D contain gratings S, and S,,,respectively. The elements of each of these detectors are essentiallyalike, both including photodetectors and relatively stationary gratings.They differ only in that the grating (S,,) orientation of D is in thecross-track direction while that of D is in the along-track direction.FIG. 8 illustrates such angular orientation for S and S,,. The outputsignals associated with |V,, and IV I have the frequencies f and frespectively.

The frequency, f is generated by the component of velocity associatedwith the backscattered granular motion which sweeps by the grating, 8,having the grat ing line spacing G Similarly, f, is generated by thecomponent of velocity associated with the backseattered granular motionwhich sweeps by the grating, S having the grating line spacing, G,,. Theparameters )1, and f,,,, are related to [V and IV I by jim- -I a.r)/ 1)and fin! nu)/ 1!) respectively, where 0.L. optical leverage aspreviously discussed.

V V cos 0 V, V sin 0 For convenience in subsequent signal processing,the grating line spaces, 6,, and G are chosen to be equal. The gratingperiod is defined as the center-to-center distance of two adjacentgrating apertures as seen in FIG. 8.

FIG. 8 illustrates a third grating, M,,, which is used in conjunctionwith S to provide the determination of drift angle, 0. Detector D whichhouses grating M is identical to detector D except that provision ismade for continuous grating translation as indicated by the arrow V inFIG. 8. A suitable means for providing this translation has previouslybeen discussed above with reference to FIGS. 3A and 4. The frequencyassociated with grating M, is generated not only by the passage ofmoving backscattered granules through the grating, but additionally byan induced change in frequency resulting from intentional gratingtranslation.

If the grating M is translated at a fixed velocity, V the centerfrequency generated by the grating after detection will be altereddepending upon the relative velocity between the target and grating, andthe target velocity direction and the grating direction of translation.

The frequency generated by the grating, M due to the grating translationalone, is Af given by fm af u where the gratings S,, and M, have thesame spacing. The frequency associated with the third detector outoutis, therefore,

fmu i fr" where f, is the frequency generated by the backseatteredgranular motion in the absence ofgrating translation. To avoid ambiguityin the phase angle 0, a restriction is imposed so that l fml lf ulGround Speed (V) The ultimate output of detector D is a signal offrequency f A D.C. voltage whose amplitude is proportional to thisfrequency is provided by a bucket counter. Subsequently, a squaringamplifier ('y=2) provides an output whose amplitude is proportional tothe square of the amplitude of its input. The output squaring amplifieris an analog of the along-track velocity squared, (V F. Similar signalprocessing of the output of detector D provides an analog of thecross-track velocity squared, (V Adding of these latter two outputswhich is then followed by square root amplification ('y= l) yields thevoltage,E,,, which is the analog of the ground speed, V, since a squareroot of the sum of two squares of vectors in quadrature has beenperformed. For bucket counters having identical conversion constants, K,(volts/Hz the output ground speed signal in units of velocity isnumerically equal to its voltage amplitude multiplied by (G /OL.) (K

Drift Angle The outputs of detectors D and D are again utilized for thedetermination of the magnitude of the drift angle. Since the output ofthe bucket counter following detector D is directly proportional to thecross-track velocity V, and since the ground velocity V has now beendetermined, the sine of the drift angle 6 is the ratio of these twovoltages. The required processing is performed by the dividing circuitry(y= l) as depicted in FIG. 7. A resolver is employed to convert sin 0 to0 (drift angle). A quadrant associated with 0, however, still requiresdetermination.

Phase of Drift Angle The phase of 0 is derived by utilizing a differenceamplifier whose two inputs are derived from the bucket counter outputsof D and D For small drift angles, the cut-on frequency of the bucketcounters in both channels 2 and 3 must equal or exceed the induced Af,,due to the added motion imposed upon M This is required so that one ofthe two possible polarities of the output of the difference amplifier isalways associated with one of th'two possible quadrants in which 6 mayoccupy.

The output of the difference amplifier can be applied to a left-rightindicator wherein a given polarity signifies a given direction; theopposite polarity signifies the opposite direction. For all cross-trackvelocities less than the minimum trackable velocity, which correspondsto Af the difference amplifier output is zero, which is the off positionof the indicator.

The absolute heading with respect to the airframe can be determined byalgebraically combining the voltage proportional to 0 and the voltageproportional to (1) provided by a reference gyro. This can beaccomplished by utilizing the polarity of the difference amplifieroutput which signifies the quadrant of 0 (drift angle).

While several embodiments of the invention have been shown and describedfor illustration purposes, it is to be understood that the invention isnot limited thereto. Various changes may also be made in the design andarrangement of the parts without departing from the spirit and scope ofthe invention as the same 'will now be understood by those skilled inthe art. For example, the embodiment of applicants invention illustratedin FIG. 7 may be modified (as shown in broken lines) to eliminate D if Dis modified by being provided with a grating which is translated part ofthe time to generate a signal with a frequency f,,,,, i Af,, and isstationary the remainder of the time to generate a signal f We claim:

1. In velocimeter comprising a source of coherent radiation directed toilluminate a surface which may move relative to the velocimeter,radiation receiving means to receive the resulting reflection ofcoherent radiation from said surface including at least one receivingaperture slit oriented perpendicular to the direction in which relativemotion is to be measured, and detecting means responsive to theintensity of the received radiation passing through said aperture todevelop a first signal having a frequency proportional to the magnitudeof the relative velocity between said velocimeter and said surface in adirection perpendicular to said slit, apparatus to provide an indicationof the direction of said relative velocity comprising:

a. means to develop a second signal which will have a frequency greaterthan that of said first signal when relative motion is in one directionand a frequency less than said first signal when relative motion is inthe opposite direction; and

b. means having as inputs said first and second signals to differencesaid first and second signals whereby an output of one polarity willrepresent relative motion in one direction and of the opposite polarityfor relative motion in the opposite direction.

2. The invention according to claim 1 wherein said at least one apertureslit comprises a first optical grating consisting of alternatelytransparent and opaque bars.

3. The invention according to claim 2 wherein said means to develop saidsecond signal comprises means to periodically move said first grating ina first direction parallel to the direction in which relative motion isto be measured whereby the signal developed at the output of saiddetecting means when the velocimeter is not moving will be said firstsignal and the signal developed while moving will be said second signal,whereby when the relative motion of said velocitometer and the motion ofsaid slit are both in first direction the frequency of said secondsignal will be lower than at the frequency of said first signal and whenthe relative motion of said velocimeter is in a direction opposite tosaid first direction the frequency of said second signal will be greaterthan that of said first signal.

4. The invention according to claim 3 wherein said first grating is inthe form of an endless belt.

5. The invention according to claim 2 wherein said means to develop saidsecond signal comprise:

a. a second grating with its bars oriented in the same direction asthose of said first grating;

b. means to move said second grating in a direction parallel to thedirection in which relative motion is to be measured; and

c. second detecting means responsive to the received radiation passingthrough said second slit to develop said second signal.

6. The invention according to claim 5 wherein said second grating is inthe form of an endless belt.

7. The invention according to claim 2 wherein said means to develop saidsecond signal comprises:

a. a second grating with its bars oriented in the same direction asthose of said first grating;

b. means to move said second grating in a direction parallel to thedirection in which relative motion is to be measured;

0. means to image the radiation passing through said second grating onthe detecting means of said first grating; and

d. means-to cause said reflected radiation to be alternately provided tosaid first and second gratings.

8. The invention according to claim 7 wherein said second grating is inthe form of an endless belt.

9; Vehicle navigation velocimeter apparatus to provide an indication ofvelocity magnitude along aircraft heading and drift velocity magnitudeand direction perpendicular to heading comprising:

a. a source of coherent radiation directed to irradiate the surface overwhich the vehicle is moving;

b. first velocimeter receiving means to receive the reflected radiationto develop a first signal having an output frequency proportional to themagnitude of the velocity component along heading;

c. second velocimeter receiving means to receive the reflected radiationto develop a second signal having a frequency proportional to themagnitude of the velocity component perpendicular to heading;

d. third velocimeter receiving means to develop a third signal having afrequency proportional to said velocity component perpendicular toheading plus a difference frequency when said velocity is in onedirection and minus said difference frequency when said velocity is inthe other direction; and

e. means to difference said second and third signals to provide anindication of the direction of the velocity perpendicular to heading.

10. The invention according to claim 9 wherein said first velocimeterreceiving means comprises:

a. a first optical grating with its bars oriented perpendicular to thedirection of heading;

b. first detecting means responsive to the received radiation passingthrough said grating to develop said first signal; and wherin saidsecond and third velocitometer receiving means comprise:

c. a second optical grating with its bars parallel to the headingdirection;

d. second detecting means responsive to the received radiation passingthrough said gratings;

e. means to periodically move said second grating in a directionperpendicular to heading; and

f. means to provide the output of said second detecting means as saidsecond signal when said grating is stationary and as said third signalwhen said grating is moving.

11. The invention according to claim 9 wherein said first and secondvelocimeter receiving means comprise respective first and secondstationary optical gratings said first grating oriented with its barsperpendicular to the heading direction and said second grating orientedwith its bars parallel to said heading direction and respective firstand second detecting means responsive to the received radiation passingthrough said gratings to develop said first and second signals and saidthird velocimeter receiving means comprises a third grating oriented inthe same direction as said second grating, means to move said thirdgrating in a direction perpendicular to heading and third detectingmeans responsive to the received radiation passing through said thirdgrating to develop said third signal.

12. The invention according to claim 11 and further including analogcomputing means to develop from said first and second signals themagnitude of ground speed, from said ground speed magnitude and saidsecond signal the magnitude of the drift angle, and from the output ofsaid differencing means the direction of drift angle relative toheading.

13. The invention according to claim 12 wherein there is available inthe aircraft an output representing aircraft heading with respect toNorth and further including means to add said drift angle magnitude anddirection to said heading to thereby develop an output representing theactual ground track with respect to North.

1. In velocimeter comprising a source of coherent radiation directed toilluminate a surface which may move relative to the velocimeter,radiation receiving means to receive thE resulting reflection ofcoherent radiation from said surface including at least one receivingaperture slit oriented perpendicular to the direction in which relativemotion is to be measured, and detecting means responsive to theintensity of the received radiation passing through said aperture todevelop a first signal having a frequency proportional to the magnitudeof the relative velocity between said velocimeter and said surface in adirection perpendicular to said slit, apparatus to provide an indicationof the direction of said relative velocity comprising: a. means todevelop a second signal which will have a frequency greater than that ofsaid first signal when relative motion is in one direction and afrequency less than said first signal when relative motion is in theopposite direction; and b. means having as inputs said first and secondsignals to difference said first and second signals whereby an output ofone polarity will represent relative motion in one direction and of theopposite polarity for relative motion in the opposite direction.
 2. Theinvention according to claim 1 wherein said at least one aperture slitcomprises a first optical grating consisting of alternately transparentand opaque bars.
 3. The invention according to claim 2 wherein saidmeans to develop said second signal comprises means to periodically movesaid first grating in a first direction parallel to the direction inwhich relative motion is to be measured whereby the signal developed atthe output of said detecting means when the velocimeter is not movingwill be said first signal and the signal developed while moving will besaid second signal, whereby when the relative motion of saidvelocitometer and the motion of said slit are both in first directionthe frequency of said second signal will be lower than at the frequencyof said first signal and when the relative motion of said velocimeter isin a direction opposite to said first direction the frequency of saidsecond signal will be greater than that of said first signal.
 4. Theinvention according to claim 3 wherein said first grating is in the formof an endless belt.
 5. The invention according to claim 2 wherein saidmeans to develop said second signal comprise: a. a second grating withits bars oriented in the same direction as those of said first grating;b. means to move said second grating in a direction parallel to thedirection in which relative motion is to be measured; and c. seconddetecting means responsive to the received radiation passing throughsaid second slit to develop said second signal.
 6. The inventionaccording to claim 5 wherein said second grating is in the form of anendless belt.
 7. The invention according to claim 2 wherein said meansto develop said second signal comprises: a. a second grating with itsbars oriented in the same direction as those of said first grating; b.means to move said second grating in a direction parallel to thedirection in which relative motion is to be measured; c. means to imagethe radiation passing through said second grating on the detecting meansof said first grating; and d. means to cause said reflected radiation tobe alternately provided to said first and second gratings.
 8. Theinvention according to claim 7 wherein said second grating is in theform of an endless belt.
 9. Vehicle navigation velocimeter apparatus toprovide an indication of velocity magnitude along aircraft heading anddrift velocity magnitude and direction perpendicular to headingcomprising: a. a source of coherent radiation directed to irradiate thesurface over which the vehicle is moving; b. first velocimeter receivingmeans to receive the reflected radiation to develop a first signalhaving an output frequency proportional to the magnitude of the velocitycomponent along heading; c. second velocimeter receiving means toreceive the reflected radiation to develop a second signal having afrequency proportionAl to the magnitude of the velocity componentperpendicular to heading; d. third velocimeter receiving means todevelop a third signal having a frequency proportional to said velocitycomponent perpendicular to heading plus a difference frequency when saidvelocity is in one direction and minus said difference frequency whensaid velocity is in the other direction; and e. means to difference saidsecond and third signals to provide an indication of the direction ofthe velocity perpendicular to heading.
 10. The invention according toclaim 9 wherein said first velocimeter receiving means comprises: a. afirst optical grating with its bars oriented perpendicular to thedirection of heading; b. first detecting means responsive to thereceived radiation passing through said grating to develop said firstsignal; and wherin said second and third velocitometer receiving meanscomprise: c. a second optical grating with its bars parallel to theheading direction; d. second detecting means responsive to the receivedradiation passing through said gratings; e. means to periodically movesaid second grating in a direction perpendicular to heading; and f.means to provide the output of said second detecting means as saidsecond signal when said grating is stationary and as said third signalwhen said grating is moving.
 11. The invention according to claim 9wherein said first and second velocimeter receiving means compriserespective first and second stationary optical gratings said firstgrating oriented with its bars perpendicular to the heading directionand said second grating oriented with its bars parallel to said headingdirection and respective first and second detecting means responsive tothe received radiation passing through said gratings to develop saidfirst and second signals and said third velocimeter receiving meanscomprises a third grating oriented in the same direction as said secondgrating, means to move said third grating in a direction perpendicularto heading and third detecting means responsive to the receivedradiation passing through said third grating to develop said thirdsignal.
 12. The invention according to claim 11 and further includinganalog computing means to develop from said first and second signals themagnitude of ground speed, from said ground speed magnitude and saidsecond signal the magnitude of the drift angle, and from the output ofsaid differencing means the direction of drift angle relative toheading.
 13. The invention according to claim 12 wherein there isavailable in the aircraft an output representing aircraft heading withrespect to North and further including means to add said drift anglemagnitude and direction to said heading to thereby develop an outputrepresenting the actual ground track with respect to North.